Method of reducing Nox compounds in flue gases of recovery boiler

ABSTRACT

A method of reducing NO X  compounds in the odor gases of a recovery boiler. In the method, ammonia in the odor gases is separated before the odor gases are combusted, the ammonia being then introduced into a recovery boiler at the pulp mill or in a separate catalyzer, where the ammonia reacts with nitrogen oxide forming water and molecular nitrogen.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method of reducing nitrogen oxide emissionsat a pulp mill in the combustion of odor gases that are released indifferent process steps at the pulp mill and contain detrimental sulphurcompounds.

At a pulp mill, in a pulp cooking process, organic nitrogen originatingfrom wood is separated in gaseous form, particularly in the form ofammonia, but also as other gaseous nitrogen compounds, during differentprocess steps, such as cooking, evaporation, stripping etc. Whenmethanol or turpentine is condensed from these odor gases, part of theammonia is also condensed into liquid form with the methanol orturpentine. When odor gases are combusted in different boilers, such asa recovery boiler, a power boiler or a separate odor gas boiler, ammoniais oxidized into various nitrogen oxides, increasing nitrogen emissionsat said mills. High phasing of air feed to reduce nitrogen oxideemissions easily results in an increase in ammonia and flue sulphuremissions. Similarly, combusting methanol or turpentine in differentboilers induces either various nitrogen oxides or ammonia emissions. Themost problematic issue is the inability to systematically avoid theformation of detrimental emissions in combustion of methanol and odorgases that contain a plurality of different compounds.

Attempts have been made to reduce emissions of nitrogen compounds, i.e.mainly nitrogen oxides, from recovery and power boilers by what is knownas stepped combustion, in which air is supplied to a boiler in severalsuccessive steps to make combustion take place mainlyunder-stoichiometric conditions. This considerably reduces the formationof what is known as thermal NO_(x). Such a solution is disclosed e.g. inSwedish Published Specification No. 468 171.

Attempts have also been made to reduce NO_(x) contents in flue gases bysupplying various reagents to flue gases for preventing the formation ofNO_(x) compounds or for converting them into a form in which they can beremoved as easily as possible, Such an additive may be ammonia or ureapurchased outside the mill, in which case the NO present in the flue gasreacts with ammonia, forming gaseous nitrogen which can be discharged tothe atmosphere. It is also possible to use various solid or liquidammonium salts as the reagent in this so-called SNCR method known perse. The problem with this technique is the high cost of reagents to bepurchased outside the mill.

It is also known to supply hydrocarbons, such as natural gas or thelike, to flue gases in a boiler, the resulting reduction in NO_(x)compounds being due to so-called hydrocarbon radicals speeding up thereactions of nitrogen compounds. The drawbacks of such methods are thehigh investment and operating costs, since the additives are purchasedoutside the mill, and in addition, equipment is required for storing,batching, adjusting and feeding the additives.

Finnish Patent Application No. 931055 discloses a method in whichoxygenous hydrocarbon, such as methanol, obtained in the pulp cookingprocess, is supplied to flue gases in a recovery boiler. In this method,methanol and any aqueous steam are supplied to the upper part of arecovery boiler to be mixed with flue gases, whereupon the flue gasesare washed with white liquor or with an aqueous solution containingammonia-based and/or alkali-based compounds. The method is based on thenitrogen oxide NO contained in the flue gases becoming partly oxidizedand forming nitrogen dioxide NO₂, which can be removed by an alkaliscrubber. The drawback of this method is that it has an effect only onthe reduction in oxides of already formed nitrogen, and the only reagentthat can be used is methanol or a corresponding hydrocarbon derivative.In addition, the method requires a flue gas scrubber suitable forremoving NO₂, and the treatment of nitrogen compounds remaining in thewashing liquid is still problematic.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of reducingthe number of nitrogen oxide and ammonia emissions readily caused bycombustion of odor gases and methanol. The method of the invention ischaracterized by separating the ammonia contained in the odor gasesbefore they are introduced into combustion.

The essential feature of the invention is that the ammonia in odourgases and methanol is separated, and, consequently, when they arecombusted, this prevents the formation of ammonia-induced nitrogenoxides or pure ammonia which would be discharged to the atmosphere withthe flue gases. In a preferred embodiment of the invention, the ammoniathus separated is supplied to a boiler in use at the pulp mill by whatis known as the SNCR method, to remove nitrogen oxides from the fluegases of said boiler. In another preferred embodiment of the invention,ammonia is separated from odor gases and/or methanol by what is known asa molecular sieve, e.g. by using zeolite.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in greater detail in the attacheddrawing which schematically shows the method of the invention as a blockdiagram.

DETAILED DESCRIPTION OF THE INVENTION

The wood to be cooked is supplied to pulp mill cooking, in which variousconstituents and wood fibres are separated therefrom. Wood fibres areled forward after various washings and black liquor remaining from thecooking is led via various process steps, such as an evaporator etc., tobe combusted in a recovery boiler. Pulp cooking and other stepsassociated with processing black liquor before it is supplied to arecovery boiler constitute a process known per se, schematically denotedin the block diagram by reference numeral 1. From this process variousodor gases are separated both in cooking and in evaporators in what isknown as stripping and super concentration, should the equipmentcomprise such a step. These odor gases are recovered and led to furtherprocessing to maintain detrimental emissions from the pulp mill as lowas possible and additionally to recover both the chemicals and thecombustion energy of the odor gases. In the final step of the pulpprocess, black liquor is sprayed to the recovery boiler 2 for combustionand recovery of the chemicals therein.

Odor gases, or at least part of them, are led to a condensing step 3, inwhich methanol is condensed into liquid form. At the same time, part ofthe ammonia is condensed with the methanol and mixed with it in a liquidform. The odor gases are then supplied to ammonia separation 4, wheregaseous ammonia is separately separated from the odor gases, whereuponthe odor gases are supplied to combustion in e.g. a recovery boiler 2.The gaseous ammonia obtained from ammonia separation, in turn, can beled either to a recovery boiler 2 or to a separate power boiler 5 to beused by what is known as the SNCR, or selective non-catalytic, method toreact with nitrogen oxides in the flue gases and to thus form molecularnitrogen and water. The ammonia is supplied to said boiler in a suitabletemperature window, which is preferably in the temperature range from920 to 950° C.

Methanol, in turn, is separately supplied to a second ammonia separationstep 6, where ammonia is separated from methanol. Methanol can then beled to combustion either in a power boiler or to be used as support fuelin a recovery boiler, an odor gas burner 7 or a separate odor gas boiler8. If desired, the previously mentioned odor gases can also be combustedeither in a power boiler or in a separate odor gas boiler. The ammoniaseparated from methanol, in turn, can be introduced in the abovedescribed manner into some boiler in use in a suitable temperaturewindow to form molecular nitrogen and water with nitrogen oxide.

Ammonia is separated in the separation step preferably by what is knownas a molecular sieve, such as zeolite, resulting in extremely pureammonia, while other impurities are led to combustion with odor gases ormethanol. The ammonia thus obtained can be used for various purposes,and, used with the SNCR method, process adjustment and, consequently,reduction in emissions can be achieved efficiently without extranon-controlled emissions.

Ammonia may also be separated from gas or methanol by a water scrubber,and in this case it naturally has to be separated from the washingsolution before further processing or use.

Ammonia may also be separated from odor gases before methanolcondensation, should this be easier from the technical point of view ofthe process. In this case only one separation step is required, andammonia does not have to be separately removed from methanol after ithas been condensed. In addition to the SNCR method, ammonia can also beused with a catalytic method, i.e. the SCR method, whereby ammonia istypically supplied to flue gases at a temperature of between 350 and400° C. to a special catalyzer, simultaneously avoiding secondaryreactions of sulphuric compounds. Furthermore, ammonia can be deliveredfrom a pulp mill to other uses or be used for the preparation ofammonium sulphate for fertilizers. In view of the economic operation ofa pulp mill, it is, however, preferable to use the separated ammonia toreduce nitrogen oxide emissions in a boiler in use at the pulp mill.

What is claimed is:
 1. A method of reducing nitrogen oxide emissions ata pulp mill during combustion of odor gases in a boiler comprising thesteps of: a) separating ammonia that exists in the odor gases directlyfrom other components of the odor gases prior to combustion of saidgases in a boiler; and b) introducing the ammonia separated in step a)to said boiler to induce removal of nitrogen oxide by selectivenon-catalytic nitrogen oxide removal.
 2. The method of claim 1 whereinammonia is separated from said odor gases in step a) by a waterscrubber.
 3. The method of claim 1 wherein ammonia is separated fromsaid odor gases in step a) by a molecular sieve.
 4. The method of claim3 wherein said molecular sieve is a zeolite.
 5. The method of claim 1further comprising separation of methanol containing a portion of theexisting ammonia from said odor gases after step a) and before step b).6. The method of claim 1 further comprising separation of methanol thatcontains a portion of the existing ammonia from said odor gases bycondensation prior to step a).
 7. The method of claim 1 wherein step b)is performed at a temperature of form about 920° C. to about 950° C. 8.The method of claim 1 further comprising separation of methanolcontaining a portion of the existing ammonia from said odor gases afterstep a) and before step b); and subsequently separating the ammonia fromthe methanol.
 9. The method of claim 1 further comprising separation ofmethanol that contains a portion of the existing ammonia from said odorgases by condensation prior to step a); and subsequently separating theammonia from the methanol.
 10. A method of reducing nitrogen oxideemissions at a pulp mill during combustion of odor gases which generatesnitrogen oxide flue gases comprising the steps of: a) passing at least aportion of odor gases generated during pulp production through acondenser to produce a methanol stream containing a first portion ofammonia from the odor gas and an odor gas stream containing a secondportion of ammonia from the odor gas; b) separating the first portion ofammonia from said methanol stream of step a), and recovering a purifiedmethanol stream and ammonia; c) separating the second portion of ammoniafrom said odor gas stream of step a), and recovering a purified odor gasstream and ammonia; and then d) introducing the first portion of ammoniaseparated in step b), the second portion of ammonia separated in stepc), or both into a recovery boiler; whereby nitrogen oxide emissionsfrom boiler flue gases are reduced by reaction of ammonia introduced tosaid recovery boiler in step d) with nitrogen oxides in said boiler fluegases.
 11. The method of claim 10 wherein said purified methanol streamof step b) is burned in a power boiler.
 12. The method of claim 10wherein said purified methanol stream of step b) is introduced into saidrecovery boiler, an odor gas burner or an odor gas boiler as fuel. 13.The method of claim 10 wherein a portion of odor gases which is notpassed through said condenser of step a) is introduced directly intosaid recovery boiler.
 14. The method of claim 10 wherein said purifiedodor gas stream of step c) is introduced into said recovery boiler. 15.The method of claim 10 wherein ammonia is separated in step b), step c)or step b) and step c) by a water scrubber.
 16. The method of claim 10wherein ammonia is separated in step b), step c) or step b) and step c)by a molecular sieve.
 17. The method of claim 10 wherein step d) isperformed at a temperature of from about 920° C. to about 950° C. 18.The method of claim 16 wherein said molecular sieve is a zeolite.
 19. Amethod of reducing nitrogen oxide emissions at a pulp mill duringcombustion of odor gases which generates nitrogen oxide flue gasescomprising the steps of: a) passing at least a portion of odor gasesgenerated during pulp production through an ammonia separator to producea purified odor gas stream and ammonia; and then, b) introducing theammonia separated in step a) into a recovery boiler; whereby nitrogenoxide emissions from boiler flue gases are reduced by reaction ofammonia introduced to said recovery boiler in step b) with nitrogenoxides in said boiler flue gases.
 20. The method of claim 19 wherein aportion of odor gases which is not passed through said condenser of stepa) is introduced directly into said recovery boiler.
 21. The method ofclaim 19 wherein said ammonia separator is a water scrubber.
 22. Themethod of claim 19 wherein said ammonia separator is a molecular sieve.23. The method of claim 22 wherein said molecular sieve is a zeolite.24. The method of claim 19 wherein step b) is performed at a temperatureof from about 920° C. to about 950° C.